Anomalous Dispersion of the Acoustic Mode in Liquid Bi

Inelastic x-ray scattering measurements on liquid Bi were carried out. Prominent acoustic mode excitations were observed in the dynamic structure factor to beyond 12 nm(-1), which resolves previously conflicting results as to their presence beyond 6 nm(-1). We find the dispersion curve of the excitation energy with momentum transfer is nearly flat from 7 to 15 nm(-1) consistent with ab initio calculations of liquid Bi [J. Souto et al., Phys. Rev. B 81, 134201 (2010)]. Our ab initio and classical molecular dynamics simulations suggest that a long-range force is needed to reproduce the flatness of the dispersion curve, and the long-range force is correlated with a local structure consisting of shorter and longer bonds in the liquid.